1. Field of the Invention
The invention relates to a viscous coupling in a torque transmitting device
2. Description of the Related Art
DE 196 45 478 A1 discloses a viscous coupling having a coupling housing and a drivable rotor, which is arranged therein and is provided for the transmission of torque via shearing areas in a working chamber filled with viscous fluid. This working chamber is partitioned off from a supply chamber for viscous fluid by a dividing wall. The viscous fluid can be directed from the working chamber via a pumping opening into the supply chamber or from the supply chamber via a feed line back into the working chamber. The feed line interacts with a control opening, which can be varied in its width by a lever of a valve operable by an electromagnet. As soon as the lever comes to bear on the control opening, the volumetric flow of the viscous fluid between the supply chamber and the working chamber ceases so that the working chamber is emptied due to the effect of the pumping opening and thus the torque-transmitting capacity decreases. At the other extreme, the lever is completely lifted from the control opening so that the working chamber can rapidly fill with viscous fluid from the supply chamber and the torque-transmitting capacity approaches a maximum. The position of this lever can be set by the electromagnet. Therefore, the electromagnet together with the valve having the lever is effective as a volumetric-flow setting means for the viscous fluid between supply chamber and working chamber.
Such a valve is relatively complicated and, on account of the lever affected by mass, sluggish, so that setting operations proceed relatively slowly. At the same time, the electromagnet, has to be very powerful on account of its distance from the lever of the valve, which distance is normally large, this configuration has unfavorable consequences for its dimensioning.
EP 0 317 186 B1 discloses a viscous coupling which does without such a valve and also (does without a supply chamber for viscous fluid. Instead, the viscous fluid, in the embodiment according to FIG. 3, is magnetorheological, with its viscosity being variable by means of an electromagnet provided on side of the viscous coupling facing a drive, such as, for example, an internal combustion engine. The torque-transmitting capacity of the viscous coupling can thus be influenced.
It is known that such a magnetorheological viscous fluid is formed by magnetizable particles, such as, for example, iron particles, being introduced into a carrier medium. When current is applied to the electromagnet, these magnetizable particles become interlinked under the effect of the magnetic field. The result of which is a high viscosity of the viscous fluid can be achieved. Conversely, however, if no torque transmission is desired at the viscous coupling, the magnetic field must be switched off in order to neutralize the effect of the magnetizable particles. However, since the working chamber has still not been emptied by this measure, the torque-transmitting capacity, although reduced, is not neutralized. The consequence of this is undesirable running of the coupling housing with the rotor during rotation of the rotor. This problem may be remedied by a very thin-bodied carrier fluid being used. Although the maximum torque-transmitting capacity of the viscous coupling is in turn considerably reduced.
A further problem turns out to be that the electromagnet must also act on the viscous fluid which is located in the part of the working chamber axially on the other side of the rotor. The electromagnet must be moved correspondingly close to the coupling housing, so that there is no possibility of forming cooling blades on the corresponding side of the coupling housing. During operation with high slip, the viscous coupling is thus constantly exposed to the risk of overheating. Furthermore, recourse must be had to a very powerful and therefore correspondingly expensive and heavy electromagnet.